OPA348AI datasheet(8/17 Pages) TI

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OPA348, 2348, 4348

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In unity-gain inverter configuration, phase margin can be

reduced by the reaction between the capacitance at the op

amp input, and the gain setting resistors, thus degrading

capacitive load drive. Best performance is achieved by using

small valued resistors. For example, when driving a 500pF

load, reducing the resistor values from 100k

Ω to 5kΩ de-

creases overshoot from 55% to 13% (see the typical charac-

teristic “Small-Signal Overshoot vs. Load Capacitance”).

However, when large valued resistors cannot be avoided, a

small (4pF to 6pF) capacitor, CFB, can be inserted in the

feedback, as shown in Figure 6. This significantly reduces

overshoot by compensating the effect of capacitance, CIN,

which includes the amplifier's input capacitance and PC

board parasitic capacitance.

FIGURE 6. Improving Capacitive Load Drive.

Normally, input currents are 0.5pA. However, large inputs

(greater than 500mV beyond the supply rails) can cause

excessive current to flow in or out of the input pins. There-

fore, as well as keeping the input voltage below the maxi-

mum rating, it is also important to limit the input current to

less than 10mA. This is easily accomplished with an input

voltage resistor, as shown in Figure 4.

OPA348

OUT

FIGURE 4. Input Current Protection for Voltages Exceeding

the Supply Voltage.

Ω

OPA348

10mA max

+5V

OUT

OVERLOAD

RAIL-TO-RAIL OUTPUT

A class AB output stage with common-source transistors is

used to achieve rail-to-rail output. This output stage is ca-

pable of driving 5k

Ω loads connected to any potential be-

tween V+ and ground. For light resistive loads (> 100k

Ω), the

output voltage can typically swing to within 18mV from supply

rail. With moderate resistive loads (10k

Ω to 50kΩ), the output

voltage can typically swing to within 100mV of the supply

rails while maintaining high open-loop gain (see the typical

characteristic “Output Voltage Swing vs Output Current”).

CAPACITIVE LOAD AND STABILITY

The OPA348 in a unity-gain configuration can directly drive

up to 250pF pure capacitive load. Increasing the gain en-

hances the amplifier’s ability to drive greater capacitive loads

(see the typical characteristic “Small-Signal Overshoot vs

Capacitive Load”). In unity-gain configurations, capacitive

load drive can be improved by inserting a small (10

Ω to 20Ω)

resistor, RS, in series with the output, as shown in Figure 5.

This significantly reduces ringing while maintaining DC per-

formance for purely capacitive loads. However, if there is a

resistive load in parallel with the capacitive load, a voltage

divider is created, introducing a Direct Current (DC) error at

the output and slightly reducing the output swing. The error

introduced is proportional to the ratio RS/RL, and is generally

negligible.

FIGURE 5. Series Resistor in Unity-Gain Buffer Configura-

tion Improves Capacitive Load Drive.

Ω to

Ω

OPA348

V+

OUT

DRIVING A/D CONVERTERS

The OPA348 series op amps are optimized for driving

medium-speed sampling Analog-to-Digital Converters (ADCs).

The OPA348 op amps buffer the ADCs input capacitance

and resulting charge injection while providing signal gain.

The OPA348 in a basic noninverting configuration driving the

ADS7822, see Figure 7. The ADS7822 is a 12-bit,

microPOWER sampling converter in the MSOP-8 package.

When used with the low-power, miniature packages of the

OPA348, the combination is ideal for space-limited, low-

power applications. In this configuration, an RC network at

the ADC’s input can be used to provide for anti-aliasing filter

and charge injection current.

The OPA348 in noninverting configuration driving ADS7822

limited, low-power applications. In this configuration, an RC

network at the ADC’s input can be used to provide for anti-

aliasing filter and charge injection current. See Figure 8 for

the OPA2348 driving an ADS7822 in a speech bandpass

filtered data acquisition system. This small, low-cost solution

provides the necessary amplification and signal conditioning

to interface directly with an electret microphone. This circuit

will operate with VS = 2.7V to 5V with less than 250µA typical

quiescent current.

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